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The Cool-Season Turfgrasses: Basic Structures, Growth and Development

Turfgrasses are narrow-leaved grass species that form a uniform, long-lived ground cover that can tolerate traffic and low mowing heights (usually two inches or below). Only a few grass species produce acceptable turf in the northern U.S. These grasses are referred to as the cool-season turfgrasses. (Warm-season turfgrasses include species that are best adapted to southern areas of the U.S. and are not discussed in this publication.) This publication covers the basic structures of grass plants, how they grow and develop, and how to identify the different species of cool-season turfgrasses.

Introduction

The study of cool-season turfgrasses begins with learning the basic structures of grass plants and how they develop from seed to mature plants. Once you understand how turfgrasses grow and develop, it is easier see how they function as long-lived communities and how they tolerate traffic, mowing, and other problems.

The Basic Structures of Grass Plants

A mature, unmowed grass plant is composed of leaves, roots, stems, and a seed head. The diagram of a grass plant in Fig. 1 shows these basic structures. Keep in mind that some grass species do not have all the structures shown and that mowed grasses typically lack flower stems and seed heads.

Figure 1. Diagram of a mature grass plant

Leaves

A grass leaf is divided into three parts: the blade, sheath, and collar region (Fig. 2). The blade is long and narrow and grows more or less horizontally away from the main shoot. The sheath is the portion of the leaf that envelopes the shoot or stem. The collar region is located where the blade and sheath meet and may or may not have structures called the collar, ligule, and auricle (Fig. 3). The smooth area on the back side of the leaf where the blade and sheath meet is the collar. It is usually a lighter color than the blade and may continue across the width of the leaf or be divided in half by a large mid vein. A ligule is a thin piece of tissue that extends just above the top of the leaf sheath and can vary in size and shape. An auricle is another small piece of leaf tissue that grows from the collar and can also vary in size and shape.

Figure 2. Diagram of a grass leaf, including blade, collar region, and sheath.

Figure 3. The collar region of a grass leaf with ligule and auricle.

Roots

Roots: Roots are the below-ground part of a grass plant that anchor it in the soil and take-up water and nutrients. Turfgrass roots are fibrous, branching, and very slender. There are two types of root systems in grasses, the primary and the secondary. The difference between the two will be explained later in this section.

Stems

Three types of stems occur in grasses; the crown, horizontal stems (rhizomes and stolons), and the flower stem. Although the crown is a stem, it does not look like the other stem types found in grasses. It is very small (just a fraction of an inch long), white, and completely enclosed by leaf sheaths. The crown is located in a protected position between the roots and shoot near the soil surface.

Horizontal stems begin to form in the crown and develop into rhizomes or stolons. Rhizomes grow below ground for a short distance, then rise to the soil surface to form new shoots. In some grass species, rhizomes produce growing points (often referred to as nodes) which give rise to roots and shoots forming new or ‘daughter’ plants. Rhizomes are usually white. Stolons grow above-ground and form nodes which give rise to new plants. Stolons are green and can creep over other grasses and bare spots in lawns, often forming circular patches.

Flower stems are also formed in the crown, usually in late spring or early summer in most cool-season grasses. Typically, they are not seen in turf since they are mowed off before they reach maturity. On unmowed grass, flower stems grow vertically and give rise to seed heads.

Seed Heads

The seed head is the flowering part of the grass plant. The basic unit of the seed head is called the spikelet. A spikelet is made up of grass flowers, the small stalks that support them, and bracts (small, papery leaves that cover the flowers) (Fig. 4). There are three types of seed heads based on the arrangement of the spikelets; panicle, spike, and raceme (Fig. 5). In the panicle type, the spikelets are borne on branches that are arranged along the central or main stem. The main stem is basically an extension of the flower stem. Kentucky bluegrass is a turfgrass with a panicle-type seed head. The spike-type seed head has spikelets that are borne directly on the main stem. Perennial ryegrass is a turfgrass with a spike-type seed head. In the raceme type, spikelets are borne on very short branches along a main stem. True raceme seed heads are rare in grasses and none of the cool-season turfgrasses produce them. Crabgrass, a common annual grass weed, has a modified spike-like raceme.

Growth and Development

Grass Seed and Seed Germination

The ‘seed’ of grass is really a dried fruit called a caryopsis (Fig. 6). The caryopsis is made-up mainly of the embryo and endosperm. The embryo contains the beginnings of the leaves, growing points, and roots of the grass plant. The endosperm makes up the bulk of the caryopsis and contains the food (primarily starch) required by the developing plant as it germinates and grows. The entire caryopsis is surrounded by the pericarp, sometimes referred to as the ovary wall (Fig. 7). The caryopsis and pericarp are enclosed by two papery structures called the lemma and palea.

The basic requirements for germination of turfgrass seed are adequate moisture, favorable temperatures, and oxygen. The first step in seed germination is absorption of water (sometimes referred to as imbibition). The rate at which grass seed absorbs water depends on the amount of water present and the permeability of the seed. As water is absorbed, the seed swells. Shortly thereafter, enzymes produced by the embryo break down the endosperm and convert the starch into carbohydrates. Carbohydrates can be used directly by the embryo and developing seedling for energy and growth.

Figure 6. Diagram of grass seed.

Figure 7. Cross-section of caryopsis including the embryo, endosperm, and pericarp.

The first evidence that the seed has germinated occurs when the embryonic root or radicle breaks through the seed coat (Fig. 8). Soon after, the first leaf emerges from the seed. At this point germination has occurred and the plant is considered a seedling.

Leaf Growth and Development

The first true leaf to emerge from the seed during germination is enclosed within a protective structure called the coleoptile. Soon after germination, the coleoptile and first leaf begin to elongate and grow towards the soil surface. The coleoptile stops growing just after it reaches the soil surface, but the leaf continues to elongate and breaks through the coleoptile sheath (Fig. 9). As the leaf expands and elongates it begins to produce its own food through a process called photosynthesis. Soon after the first leaf emerges, the developing seedling produces a second leaf from the growing point or node enclosed in the coleoptile. All succeeding leaves follow the same route — emerging from the growing point and growing upward within the folds of the older leaves. Eventually, the coleoptile withers away and is no longer visible.

Figure 9 Turfgrass seedling

The growing point that gives rise to leaves on mature turfgrass plants is at the tip of the crown and is called the stem apex. This structure looks like a small dome with ridges rising alternately from each side (Fig. 10). These ridges are the beginnings of the new leaves. As a leaf begins to develop, it encloses the entire stem apex. This leaf continues to elongate and expand and eventually forms a fully-developed leaf with a blade, sheath, and collar region. The fact that grass leaves begin to grow from the stem apex located at the base of the plant is the main reason why grass can be mowed without sustaining serious injury. Growth continues from the base of the leaf after a portion of the leaf blade is mowed off.

Figure 10. Stem apex of grass plant. The ridges are the beginning of new leaves. As a leaf begins to develop, it encloses the stem apex.

New leaves are produced from other ridges on the stem apex and emerge from the folds of the older leaves. Thus, the oldest leaves are on the outside of the plant and the youngest are located in the center of the plant. Turfgrass leaves live for a period of time then die and are replaced by new ones. Under favorable environmental conditions, the number of leaves per plant remains the same as new leaves replace those that die.

The rate of leaf growth is dependent on many factors including temperature, moisture, nutrition, and to some extent, daylength. Optimum temperatures for leaf growth among the cool-season turfgrasses range from 60° to 75°F. Leaf growth increases with increasing daylength as long as temperatures are within the optimum range and moisture is adequate. Application of nitrogen fertilizer can greatly increase leaf growth if moisture and temperature are not limiting.

Root Growth and Development

Soon after the radicle emerges from the seed, the first true roots develop from the embryo. These roots are called primary roots and begin taking-up water and nutrients from the soil when they are fully developed. Although the primary roots continue to function for up to a year after germination, water and nutrient uptake is gradually taken over by the secondary roots (sometimes referred to as adventitious roots) which become more numerous as the grass plant matures. Secondary roots are produced from nodes in the crown or from nodes on horizontal stems.

Turfgrass roots are very different from leaves and stems (Fig. 11). The growing point or meristem is located at the tip of the root. This is where all new root cells are produced. The meristem is protected from the abrasive effects of the soil by a structure called the root cap. In the area just behind the meristem, new cells grow mostly in length. This area is called the region of cell elongation. Behind the region of cell elongation, cells begin to develop into tissues that absorb water and nutrients. Among these tissues are root hairs — tiny hair-like outgrowths that grow from the root surface into the surrounding soil. The primary function of root hairs is water and nutrient uptake. Root hairs number in the billions for a fully-developed root system and can greatly increase the amount of soil the roots contact. Water and nutrients are transported from root hairs to the interior of the root where special conducting tissues move water and nutrients to the leaves and shoots.

Turfgrass root growth is affected mainly by soil temperature, moisture, and oxygen. The optimum temperatures for root growth of cool-season grasses are lower than those for shoot growth. Although the optimum temperature range for rooting differs somewhat among turfgrass species, most cool-season turfgrasses produce the best root growth at soil temperatures between 50° and 65°F. When temperatures reach 90°F in the surface inch of soil, Kentucky bluegrass root growth is greatly reduced. Roots of cool-season grasses can grow at soil temperatures below 50°F, but growth slows dramatically as temperatures approach freezing (32°F). Root growth is greatest for cool-season grasses during spring and fall and much reduced during the summer and winter months.

Turfgrasses take-up water from the soil through their root system. The amount of water the roots absorb will depend primarily on the number of roots, the depth of rooting, and the amount of water in the soil. Since the rooting depth of cool-season grasses is usually between 2 and 6 inches, most water absorption initially occurs near the soil surface. As the surface water is depleted, roots begin using up water deeper in the soil. A well-developed and actively-growing root system can take advantage of this deeper soil moisture as surface moisture is depleted in dry periods. Contrary to popular belief, roots do not ‘seek out’ water, instead they grow more vigorously and proliferate where water is available.

Turfgrass roots need an adequate supply of oxygen for normal growth and development. Severely compacted soils have limited supplies of oxygen and will not support good root growth even when favorable temperatures and moisture levels are present. Too much water will also deplete the soil of oxygen and cause deterioration of turfgrass roots. Soils with loose, crumbly structure and good drainage are ideal for root growth and development.

Other factors that have an effect on root growth and development are soil pH, fertilization practices, salt concentrations, herbicides, diseases, and insects. These will be discussed in other sections of this manual.

Stem Development and Tillering

Of the three stem types mentioned previously, the crown is the most important. It gives rise to leaves, secondary roots, and other stems. Because new leaf growth occurs at the base of the plant, grass plants can tolerate mowing and some other types of minor injury to leaf blades. However, crowns can be damaged by mowers when blades are set too low. When this happens, plants are severely damaged and new leaf growth is unlikely.

Since new secondary roots are produced from the crown, some of the existing root system can be damaged without killing the plant — provided that the root-initiating portion of the crown is not injured. Sod producers routinely sever a portion of the grass root system with sod harvesters, then transport the sod to a new location. The newly-laid sod generates a new root system from secondary roots formed in the crown.

Rhizomes and stolons begin to grow from nodes in the crown and break through the surrounding leaf sheaths to spread laterally. Rhizomes of Kentucky bluegrass and creeping red fescue grow beneath the soil surface and then turn up towards the soil surface to form new shoots (Fig. 12). Some other grasses (mostly warm-season grasses and weed grasses) have long rhizomes that produce nodes that can branch and produce shoots and roots, forming new plants.

Figure 12. Rhizomes of Kentucky bluegrass

Rhizomes are a desirable trait in turfgrasses because they allow plants to send new shoots into areas that are thin or damaged by traffic, drought, and/or disease. Kentucky bluegrass is the premier sod grass in the northern U.S. because its rhizomes allow turf to ‘knit’ and hold together as the sod is cut, rolled, and lifted. Kentucky bluegrass is a desirable species for use in athletic fields because its rhizomes provide superior footing for athletes.

Stolons grow along the soil surface and can creep over established turf (Fig. 13). New shoots are produced from nodes or from tips of the stolon as it turns upward. Although the stoloniferous cool-season turfgrasses, rough bluegrass and creeping bentgrass, are desirable for some applications, they can be very troublesome weeds if mixed with other lawn grasses since they form light-colored, circular patches as they creep over the more desirable turfgrasses.

Figure 13. Stolons of creeping bentgrass

Tillers are shoots that develop from crown tissues and grow vertically within the sheaths that surround the crown (Fig. 14). Mature tillers produce leaves, stems, and root systems; thus, they can function independently of the mother plant. Tillers increase the shoot density of lawns by replacing shoots that die in winter and summer. Individual tillers live for about a year and formation of new tillers is stimulated by cool temperatures, short daylengths, moderately low mowing heights, and high mowing frequencies. Peak tiller formation occurs in early spring and fall. Turfgrass stands are long-lived because dying shoots are constantly being replaced by new tillers. This process is so gradual that the transition is unnoticeable.

Figure 14. Two tillers developing from the crown of a grass plant.

Carbohydrates — The Real Plant Food

Lawn fertilizers are often marketed as ‘plant food’. Although most people realize that nutrients from fertilizers are required by plants for proper growth and development, they may not realize that fertilizers are not really plant food. Plants make their own food through photosynthesis, a chemical reaction in leaves involving water, carbon dioxide (CO 2 ) and light energy. The end products, carbohydrates, are used by plants for energy and growth and are the true plant food.

Carbohydrates can be stored in stem and crown tissues when they are made faster than they are used. Storage is greatest in fall and is beneficial since the plant needs carbohydrates for recovery from injury when turf is damaged by pests, drought, heat, and mower injury the following year. Depletion of carbohydrates is fastest in spring, especially under low mowing heights and high nitrogen fertility. If depleted too quickly, the turf may go into the summer months in a weakened state. This is one reason why turfgrass professionals do not apply excess amounts of nitrogen and mow below optimum heights of cut in spring.